With the completion of human genome project, there is now a focus on developing new DNA sequencing technology that will reduce the cost of sequencing dramatically without sacrificing accuracy, which will ultimately enable personalized medicine in healthcare (1). Current state-of-the-art DNA sequencing technology faces limitation in terms of cost, read-length, and throughput. In this regard, DNA sequencing by synthesis (SBS), where the identity of each nucleotide is detected immediately after its incorporation into a growing strand of DNA in a polymerase reaction, offers an alternative approach to address some of these limitations. An important requirement for the SBS approach is a 3′-OH capped fluorescent nucleotide that can act as a reversible terminator (2), where after the identification of the nucleotide incorporated in a DNA polymerase reaction, the 3′-OH capping group along with fluorescent label are removed to regenerate a free 3′-OH group thus allowing DNA chain elongation. The importance of removing the fluorescent label after each base identification is to make sure that the residual fluorescence from the previous nucleotide incorporation does not affect the identification of the next incorporated fluorescent nucleotide.
The speed and sequence read length of SBS depend on the yield of the cleavage efficiency of the fluorophore and the allyl group. Due to multiple steps required in the identification, removal of fluorescent label, and regeneration of 3′-OH group after each nucleotide incorporation in SBS, the loss of even a minor efficiency at each step may lead to inhibition of prolonged read length. For this reason, any improvement in efficiency within each cycle of nucleotide identification, fluorophore removal, and 3′-OH regeneration can have significant impact on read length, thus tackling the physical limits in DNA sequencing by synthesis.
Summary
This invention provides a nucleotide analogue comprising (i) a base selected from the group consisting of adenine or an analogue of adenine, guanine or an analogue of guanine, cytosine or an analogue of cytosine, thymine or an analogue of thymine and uracil or an analogue of uracil, (ii) a deoxyribose, (iii) an allyl moiety bound to the 3′-oxygen of the deoxyribose and (iv) a fluorophore bound to the base via an allyl linker.
This invention also provides a method for making a nucleotide analogue wherein the nucleotide analogue comprises (i) a base selected from the group consisting of adenine or an analogue of adenine, guanine or an analogue of guanine, cytosine or an analogue of cytosine and uracil or an analogue of uracil, (ii) a deoxyribose, (iii) an allyl moiety bound to the 3′ oxygen of the deoxyribose, and (iv) a fluorophore bound to the base via an allyl linker which is not an iso-allyl linker, comprising the steps of:                (a) contacting 6-amino-hex-2-en-1-ol and an N-hydroxysuccinimide ester of a fluorophore in the presence of a first suitable solvent and a suitable base;        (b) treating the resulting product of step (a) with DSC/Et3N in a second suitable solvent; and        (c) treating the resulting product of step (b) with a 3′-O-allyl-dNTP-NH2 in the presence of a suitable buffered solvent, wherein the base of the 3′-O-allyl-dNTP-NH2 is an adenine, guanine, cytosine, uracil, or an analogue thereof, thereby making the nucleotide analogue.        
This invention also provides a method for making a nucleotide analogue wherein the nucleotide analogue comprises (i) a base selected from the group consisting of adenine or an analogue of adenine, guanine or an analogue of guanine, cytosine or an analogue of cytosine and uracil or an analogue of uracil, (ii) a deoxyribose, (iii) an allyl moiety bound to the 3′ oxygen of the deoxyribose, and (iv) a fluorophore bound to the base via an allyl linker, comprising the steps of:                (a) contacting 2-(2-amino-ethyl)-prop-2-en-1-ol and an N-hydroxysuccinimide ester of a fluorophore in the presence of a first suitable solvent and a suitable base;        (b) treating the resulting product of step (a) with DSC/Et3N in a second suitable solvent; and        (c) treating the resulting product of step (b) with a 3′-O-allyl-dNTP-NH2 in the presence of a suitable buffered solvent, wherein the base of the 3′-O-allyl-dNTP-NH2 is an adenine, guanine, cytosine, uracil, or an analogue thereof, thereby making the nucleotide analogue.        
This invention also provides a method for determining the sequence of a DNA comprising performing the following steps for each residue of the DNA to be sequenced:                (a) contacting the DNA with a DNA polymerase in the presence of (i) a primer and (ii) four fluorescent nucleotide analogues under conditions permitting the DNA polymerase to catalyze DNA synthesis, wherein (1) the nucleotide analogues consist of an analogue of dGTP, an analogue of dCTP, an analogue of dTTP or dUTP, and an analogue of dATP, (2) each nucleotide analogue comprises (i) a base selected from the group consisting of adenine, guanine, cytosine, thymine or uracil, and analogues thereof, (ii) a deoxyribose, (iii) an allyl moiety bound to the 3′-oxygen of the deoxyribose and (iv) a fluorophore bound to the base via an allyl linker, so that a nucleotide analogue complementary to the residue being sequenced is bound to the DNA by the DNA polymerase, and (3) each of the four analogues has a predetermined fluorescence wavelength which is different than the fluorescence wavelengths of the other three analogues;        (b) removing unbound nucleotide analogues;        (c) determining the identity of the bound nucleotide analogue; and        (d) following step (c), except with respect to the final DNA residue to be sequenced, chemically cleaving from the bound nucleotide analogue the fluorophore and the allyl moiety bound to the 3′-oxygen atom of the deoxyribose,        thereby determining the sequence of the DNA.        
This invention also provides a kit for performing the instant method comprising, in separate compartments,                (a) a nucleotide analogue of (i) GTP, (ii) ATP, (iii) CTP and (iv) TTP or UTP, wherein each analogue comprises (i) a base selected from the group consisting of adenine, guanine, cytosine, thymine or uracil, or an analogue thereof, (ii) a deoxyribose, (iii) an allyl moiety bound to the 3′-oxygen of the deoxyribose and (iv) a fluorophore bound to the base via an allyl linker,        (b) reagents suitable for use in DNA polymerization; and        (c) instructions for use.        
This invention also provides a method for covalently affixing a detectable moiety, via an allyl linker, to an NH2-bearing molecule, comprising contacting the detectable moiety with the NH2-bearing molecule in the presence of a suitable solvent and suitable base, wherein the detectable moiety comprises a mass tag, fluorophore or chromophore bound to a NHS ester of an allyl moiety. In one embodiment of the NH2-bearing molecule is a nucleotide and the detectable moiety comprises a fluorophore.